Anselm and Aquinas

Posted: Wed, Sep 24, 2025

Background: The Arabic-Latin translation movements, Islamic philosophers exposing the Latin West to ancient Greek philosophy and profoundly shaping the medieval Christian world.

Two kinds of knowledge:

  • A priori: Can be justified without appealing directly to experience.
    • Example: Freedom of speech includes a right to protest.
    • The ontological argument seeks to demonstrate God’s existence a priori.
  • A posteriori: Cannot be justified without appealing directly to experience.

Anselm’s Ontological Argument

Reconstruct the core argument: Pp. 99–100.

For Anselm, God =df that than which nothing greater can be thought.

  1. God does not exist in reality. (According to the Fool)
  2. God exists in understanding. (Even for the Fool)
  3. Existing in reality is greater than existing merely in understanding.
  4. So, if God exists in understanding but not in reality, then it’s possible to think of something greater than God. (Implied by 3)
  5. So, it’s possible to think of something greater than God. (Follows from 1, 2, 4)
  6. But God is that than which nothing greater can be thought.
  7. Contradiction. (Follows from 5, 6)
  8. Therefore, reject 1: God exists in reality.

Reductio ad absurdum: P leads to absurdity; therefore, not P.

The crucial move: A perfect being’s essence implies its existence.

Gaunilo’s perfect island

Perfect Island =df that than which no greater island can be thought.

Anselm & Ibn Sina

God’s existence is necessary, not contingent: if God exists at all, he must exist necessarily.

xkcd comics: ... But wouldn't a God who could find a flaw in the ontological argument be even greater?

xkcd comics: A God who holds the world record for eating the most skateboards is greater than a God who does not hold that record.

Aquinas’ Cosmological Arguments

Let’s focus on the first mover argument: P. 15.

  1. Some things are moved.
  2. Everything that is moved is moved by something else.
  3. This chain cannot go back forever.
  4. Therefore, there must be a first mover not moved by anything else.
  5. An unmoved first mover has to be God.
  6. Therefore, God exists.

Weak points:

  • Against (2): Self-moved movement?
  • Against (3): Can’t the chain go back forever?
    • Eternal chain: Aristotle.
    • Circular chain: A moved by B moved by C moved by A…
  • Against (5): Does the unmoved first mover have to be God?
    • Is the unmoved first mover all-powerful? all-good? all-knowing?